Nanostructure and Defects Tailoring of Fe Oxide Catalysts for Nitrogen Reduction Reaction

Electrochemical ammonia synthesis under mild conditions is considered a promising strategy to produce green ammonia for renewable energy storage, taking advantage of its high hydrogen content and easy liquefaction. However, the use of electrochemical processes for ammonia synthesis has been so far challenging due to the absence of efficient catalysts, able to break the N₂ triple bond, and to the competition with the more favorable hydrogen evolution reaction in aqueous solution, preventing the nitrogen reduction to ammonia. In this work we adopt several advanced characterization techniques to optimize the efficiency of Fe oxide nanoparticles as a sustainable, earth-abundant catalyst for nitrogen reduction. SEM, XPS and TEM analyses with electron energy loss spectroscopy (EELS) have been employed to compare different deposition conditions and to optimize the morphology and composition of the nanoparticles, allowing higher catalytic activity toward electrochemical ammonia synthesis. The produced ammonia has been determined through the indophenol blue method, adopting a rigorous experimental protocol to avoid environmental contaminations. The structural characterization of the catalyst has been coupled to an electrochemical activation process, which is effective in increasing the electrochemical active area. XPS analyses performed after the electrochemical activation have shown that the increase of available active sites corresponds to an increase of oxygen vacancies and of the Fe⁺²/Fe⁺³ ratio. After the activation process, the faradic efficiency for nitrogen reduction increases from 3.8 % to 20.4 %, giving insight on the physical mechanisms which determine the performance of FeOx catalysts for ammonia synthesis.